Modifying the mix

Asphalt plant manufacturer Ciber looks at the influence of modified asphalt cement on the quality of the mix and production in an asphalt plant.According to asphalt plant and equipment manufacturer Ciber Equipamentos Rodoviários (Ciber), the new inputs available for the production of hot milled asphalt concrete are providing opportunities to create special mixes with properties more relevant to the complexities of paving works.

Marcelo Zubaran, Product and Application Engineer at Ciber says one of the main highlights in this space is petroleum asphalt cement. It’s a product that comes from refineries with properties related to petroleum, but is insufficient when it comes to dealing with road traffic and climate conditions.

He says additives such as polymers and rubber dust improve the performance of this type of binder, but may also affect its production in an asphalt plant.

“As such, knowledge about the relationship between inputs, the project and the respective impacts on production may define the quality of the final product.”

Mr. Zubaran says asphalt cement is considered to be the residue of petroleum distillation, which comes from lighter gases and oils evaporating and being collected during the process in the refinery. This binder gives the asphalt mix flexibility and impermeability, which are necessary to deal with the dynamics of loading and climate conditions.

“However, certain rheological properties of this binder, such as viscosity, softening point, elastic recuperation and thermal susceptibility, may be insufficient to a certain traffic volume or climate of the place of application. In those cases, modified asphalts stand out as the solution,” explains Mr. Zubaran.

He says that modified asphalt allows for the development of special projects such as a draining asphalt coating and mixes with high superficial adherence.

“In general, modified asphalt is more viscous and, for this reason, may have different impacts on the plant technology during production.”

The modifying additive is mixed into the binder, normally through the provider, and goes through specific processes to fuse the additive and the asphalt.

The polymers present a higher fusion capacity than the rubber dust and it can be measured through a phase separation test.

When the phase separation is higher than 5 per cent, Mr. Zubaran says, it is recommended to introduce stirrers in the tank with rotation higher than 100 revolutions per minute to avoid decantation of the additive. If the phase separation is lower than 5 per cent, a recirculation system through a pump is enough to keep the product homogeneous.

“In general terms, rubber asphalt requires stirrers and the polymer requires recirculation only, meaning it’s possible to apply the stirrers to all binders as they present higher shearing in the mix inside the tank,” explains Mr. Zubaran. “Furthermore, stirrers and re-circulators aid in the homogenisation of asphalt temperature inside the tank and speed up the heating of the binder, thus reducing fuel consumption used in heating up the asphalt.”

Regarding the quality of the asphalt mix, he says it is fundamental that the binder is heated for the mix with other aggregates according to their viscosity.

“Variations in asphalt temperature in the mix may damage the adhesiveness of the binder with the aggregates or accelerate asphalt ageing. It is also important to begin the compaction of asphalt concrete according to the binder viscosity.”

According to Mr. Zubaran, the combination of modified asphalt and aggregates should happen in a mixer that gives the mix a high shearing rate.

“The mixing time should be adjusted a number of times so that the most viscous binder may disperse and cover all surfaces of the aggregates.”

Certain mixes also require a dry mixing stage so that the thickness of the binder pellicle is homogeneous on all faces of the aggregates. Mr. Zubaran says those technological capabilities may be found in asphalt plants with a pug mill mixer in continuous production flow or per batch.

“Regarding the plant’s maintenance, it is important to constantly verify the conditions of the asphalt filter of the tank in producing modified asphalt. There may be an accumulation of material in the filter, resulting in an increase of pressure in the pump. As a consequence of that, the pump flow may be reduced.”

Mr. Zubaran explains that another system that requires monitoring in those applications is the system for asphalt binder spreading. If the asphalt viscosity is too high, it’s recommended to remove the injector nozzles to stop the nozzles from blocking.

“The asphalt plant should cater to the production of the most varied asphalt mixes. The tuning between the criteria of operation and maintenance and the inputs used then help add to a project’s success.

“Modified asphalt is a great technical alternative and should be applied in harmony with the equipment.”

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